1. Field of the Invention
The present invention relates to a semiconductor memory device and manufacturing method thereof, and more particularly to a nonvolatile semiconductor memory device for storing information in a charge storage layer, and manufacturing method thereof.
2. Description of the Related Art
In a nonvolatile semiconductor memory device such as a floating gate type nonvolatile semiconductor memory device or a metal-oxide-nitride-oxide-semiconductor (MONOS) type nonvolatile semiconductor memory device, information charge is stored in a charge storage layer. Miniaturization of a semiconductor device has resulted in a reduction in the amount of charge stored in the charge storage layer. Thus, to secure data retention characteristics of the nonvolatile semiconductor memory device, it is important to suppress leakage of information charge from the charge storage layer.
For example, the floating gate type nonvolatile semiconductor memory device includes a floating gate electrode for storing charge and a control gate electrode for controlling an operation of the semiconductor device. An interelectrode dielectric (referred as an interpoly dielectric, hereinafter) is formed between the floating gate electrode and the control gate electrode. In a conventional nonvolatile semiconductor memory device, for example, a so-called ONO film constituted of a first silicon oxide (SiO2) film, a silicon nitride (Si3N4) film and a second SiO2 film has widely been used as the interpoly dielectric.
If charge stored in the floating gate electrode leaks to the control gate electrode through the interpoly dielectric, the data retention characteristics of the nonvolatile semiconductor memory device is deteriorated.
One of technologies to improve leakage characteristics of the interpoly dielectric is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-288412. According to this technology, an ONO film which is an interpoly dielectric includes an SiO2 film formed by thermally oxidizing a floating gate electrode, an Si3N4 film formed by chemical vapor deposition (CVD), and an SiO2 film formed by thermally oxidizing a silicon-containing film, such as an SixNy film (x≠3, y≠4), formed by CVD.
One of technologies suitable for forming a thinner interpoly dielectric is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-213820. According to this technology, the interpoly dielectric is formed as follows. First, a silicon oxide film is formed on a floating gate electrode by CVD. Then, a part of the silicon oxide film is converted into a silicon oxynitride film by heat treatment in an atmosphere containing nitrogen monoxide. As a result, an interpoly dielectric having a larger dielectric constant is formed. A nonvolatile semiconductor memory device that uses this interpoly dielectric has excellent charge retention characteristics because it can suppress a leakage current.